1. Field of the Invention
The present invention relates to an electrophotographic recording apparatus which records externally-supplied electical signal information on a recording sheet as of plain paper by electrophotographic processing, and more particularly to a method of controlling the toner concentration of developer so as to maintain constant the stable quality of an image produced on the recording paper.
2. Prior Art of the Invention
In apparatus such as electrophotographic printers, compound information recording apparatus, facsimile machines, and the like which record external information by an electrophotographic process, a visible image recorded on a sheet of recording paper is obtained by developing externally-supplied information formed on a photosensitive member with a developer consisting of toner particles and carrier particles. In order to maintain the quality of the recorded image and, particularly a constant density of the image, the concentration of the developer (i.e. of the toner thereof) must be kept substantially constant. Various methods of detecting such toner concentration have heretofore been suggested, such, for example, as an optical method by which the amount of light transmitted through a transparent disc to which toner is attached; is converted to an electric current an electrical method by which the variation of electric current flowing through the developer is measuring (taking advantage of the insulation characteristics of the toner) and a magnetic method by which the magnetic permeability of the developer is measured (based upon known variation thereof as the toner concentration in the developer varies). On the other hand, insofar as controlling the toner concentration in an information recording apparatus utilizing an electrophotographic process, only a method similar to the aforementioned optical method is understood to have been suggested.
The practice of such any of such conventional toner concentration detecting methods, requires a special detecting element and detecting circuit requiring space therefor within the recording apparatus, thus increasing the size and cost of such apparatus.